Sound recording apparatus



NOV. 10, 1942. J GEHMAN SOUND RECORDING APPARATUS Filed Aug. 29, 1941 2 Sheets-Sheet 1 .Q MST IDDDDDD velope of the recorded sound waves.

the amplifier tended to clip the peaks. disadvantage has been that with the more usual Patented Nov. 10, 1942 V 2,301,268 soUNp RECORDING APPARATUS John B. Gehman, Indianapolis, Ind, assignor to Radio Corporation 01' America, a corporation 01'. Delaware Application August 29. 1941, Serial No. 408,866

6 Claims.

This invention relates to sound recording apparatus and more particularly to a ground noise reduction amplifier for use in recording sound on film' and is an improvement on the amplifier of my application No. 362,581, filed October 24, 1940. It has heretofore been proposed, a for example in McDowell Reissue Patent 21,389, to reduce the ground noise produced by the transmission of light through the transparent portion of a sound record film by rendering that transparent portion opaque over an area corresponding to the en- 'I'his is ordinarily done as described in the said McDowell patent by interposing a shutter in the recording light beam and causing the shutter to follow the envelope of the sound waves being recorded so as to leave a corresponding transparent area on the negative. This sound record which when printed onto the sound record produces a correspending opaque area. In the recording oi variable density sound records a corresponding procedure has been used by varying the opening of the light valve or intensity or the glow lamp as the case may be, so that at low sound intensities the average light transmitted to the negative is comparatively little, increasing with the sound amplitude and thereby increasing the average density of the negative sound record with amplitude and conversely reducing the average density and light transmission of the positive soun record at low amplitudes;

The amplifier shown in the McDowell patent was reasonably satisfactory for the purpose but improvements in the standards of quality 01 sound recording and reproduction have been so great that improvement in the ground noise reductlon amplifier has been continually required.

that of my earlier application in that, on the application of a signal within predetermined lim its, the ground noise reduction device is opened by a predetermined amount and stays at that opening for a predetermined time unless a signal of greater amplitude should follow within that time and cause further opening, as in the earlier application, but the limits and holding time are both adjustable.

This is accomplished by the use of a series of gas tubes which operate at diiierent potentials. The use of gas tubes operating at different potentials to secure a step by step control is broadly shown in Strieby Patents Nos. 1,776,821 and 1,776,822, issued September 30, 1930. In those patents, however, neon tubes are used for the purpose which require the application oihigh voltages to secure their operation, and the currents passed through the tubes are used to actuate the controlled mechanism.

In this apparatus, I provide an arrangement of gas filled hot cathode tetrodes which are used to control the vacuum tubeamplifiers supplying operating current to the ground noise reduction shutter. With this arrangement, the gas tubes may control the timing independently of the amplitude oi the currents controlled and they may be set; to operate at much lower voltages than required by the tubes of the Strieby patents so that a much greater range ofvolume may be controlled. The amplifier tubes connected to' the successive gas tubes are arranged to control an 4 improved type of direct current amplifier which One deficiency of early types of amplifiers has been that they tended to follow the wave form of the sounds being recorded too closely. Another disadvantage has been that when a sound consisted of a series of peaks spaced at intervals large in proportion to the timing of the peaks Another types of amplifiers it has been diilicult to provide a device which would open rapidly and close slowactuates the ground noise reduction apparatus.

One object of the invention is to provide an improved type of ground noise reduction timing circuit.

Another object of the invention is to provide an improved control circuit which is capable or any required adjustment as to time constants,

Another object of the invention is to provide an improved amplifier having an A.-C. input and a D.-C. output.

Another object of the invention is to provide such an amplifier which will have greater stabil ity than that described in my prior application.

Another object of the invention is to provide such an amplifier which will be simpler than that described in myprior application.

Another object of the invention is to provide such an amplifier in which the holding time of the several steps is separately adjustable.

Other and incidental objects of the invention will be apparent to those skilled in the art from a reading of the following specification and an inspection of the accompanying drawings in.

a which:

maximum input corresponding to 100 percent modulation of the sound track. In this amplifier, the speech input is applied through the transformer 9 across the 100,000 ohm potentiometer it, which determines the portion or the input signal applied to the control grid 01' the tube II which is preferably of the commercial type known as the 6J7. This tube is provided with a cathode resistor 12 having a value of approximately 470 ohms and with a screen dropping resistor 13 of 1,200,000 ohms bypassed by the capacitors 23 and 23' of .5 and 2 mid. respectively. The 220 volt positive potential is supplied to the screen and plate through the 15,000 ohm resistor l5 and is fed to the plate through the 220,000 ohm resistor M. The output from the tube H is applied to the tube I I through the 0.1 mid. condenser 24, grid bias being applied through the 390,000 ohm resistor I.

The tube 59, which is preferably of the type known as the 61"6, is provided with the usual cathode biasing resistor 20 680 ohms. Degeneration is provided by feedback from the plate or the tube I8 through the resistor 8 having a resistance or 100,000 ohms and the filter composed of the resistor i6 01' 180,000 ohms and the capacitor i'l having a value of .025 mid. The output of the tube it is fed to the transformer 2| through the 1.0 mfd. capacitor 06, and thence to the input of the step amplifier. Direct current at a potential of 220 volts is supplied through the power supply terminals 21 and 20 and is fed to the plate of the tube i0 through the reactor 25 having an inductance of 40 henrys at 18 m. a. D. C. The output from the speech amplifier is applied across the five potentiometers 40, M, H and BI in the five stages of the step amplifier. It will be apparent to those skilled in the art that a greater or lesser number of stages may be used according to the number of steps required.

Between the potentiometer-s and the gas tubes 30, Si, 32, 33, M there are provided 1 megohm protective resistors 42, 53, 63, i2, 02 so that un usually high signals will not injure the tubes and so that a maximum signal will not injure the tube adjusted to respond to the minimum signal.

The tubes 30, Si, 32, 33, 34 are preferably gas tetrodes of the type commonly known as the RCA 2051, although the RCA 2050 tubes may be used. These tubes in the circuit shown will trigger at a voltage of approximately two volts on tiaagrid, and the potentiometers are so adjusted in the successive stages that this voltage is pplied at the appropriate volume level in the recording apparatus. The second grid of each of these tubes is connected to the: cathode as shown.

Potential is applied to the plate of the tubes 3., 3i 3!, 33, 34 through the resistors I5, 58, 56, II and 85, each of 330,000 ohms.

The present circuit difiers from that of my earlier application, among other things, in the timing circuit. In the circuit of my earlier application the timing was controlled entirely by the resistors corresponding to 15, 58, etc. In the present circuit I provide a resistor connected between the plate and grid return of each of the gas tubes in shunt to the timing capacitors and this resistor may be made adjustable to control the timing of the individual stages.

The stage marked A will first be described in detail. In this stage the potentiometer 40 has a resistance or 250,000 ohms while the fixed resistor ll has a resistance of 220,000 ohms, the resistor 42 having a resistance of one megohm as above described. The capacitor 44 has a ca-v pacitance or .025 mid. and the resistor 43, which may be variable, has a resistance of 6.8 megohms. When the tube 30 is triggered the capacitor 44 discharges through the resistor 04 of 2200 ohms which determines the discharge time, which, in

the present instance, is approximately microseconds, to ten percent of charge at which point the resistor 05 causes such a voltage drop that the necessary drop of 14 volts across the tube 30 cannot be maintained and the tube then becomes non-conductive. After the discharge the capacitor H starts to charge through the resistor 40 but it between this time and full charge another impulse is recelved it will again discharge the capacitor 44 down to 14 volts. This arrange-.

ment causes a sawtooth wave to be applied to the grid of the tube 30 as shown in Fig. 3 of the draw- Due to the fact that the tube 30 being a gas tube is either conductive or non-conductive, the wave shape produced by the output of the tube 10 and therefore by the output of the tube 35 is that shown in Fig. 4. It will be apparent that if a second impulse occurs during the hold-over time indicated in Fig. 4 it will move the beginning of the closing time to the right in that figure without aflecting the amplitude of the openin on that particular stage.

The timing oi the tube 3| is determined by two different factors, the charging time of the capacitor 44 is determined by the resistors 43 and 05 acting as if they were in parallel and variation of the capacitor 43 will correspondingly aflect the charging time. In addition to this effect the resistor 43 changes the grid bias on the tube 30. Decreasing the value of the resistance will.

increase the bias, making the operating point nearer cut-off and increasing the holding time while at the same time, as pointed out above, the decrease in the value of this resistor will decrease the charging time.

Although the opening time of the individual stage is approximately 120 microseconds as described above, the cpening time applied to the ground noise reduction apparatus is determined by the filter circuit 80, 9| described in more detail hereinafter.

When no signal is applied to the apparatus the potential in the capacitor 44 is built up to its maximum and the tube 35 is biased'to its normal current by a negative grid bias of five volts. When a signal is applied sufilcient to trigger the tube, the tube 30 immediately becomes highly conductive and the capacitor 44 discharges through the tube 30, as described above. This promptly raises the negative grid potential on the tube 35 considerably beyond the cut-oil of the tube 35 and causes the corresponding opening of the ground noise reduction shutter or other device through the D. C. amplifier I00.

As.pointed out above, each of the tubes 35, etc., is biased considerably above cut-oi! when the corresponding tube 30 is triggered. Due to this increased bias, the tube 35 does not begin to con.- duct until the charge on the corresponding capacitor 44 has built up to an appropriate value through the resistor 45. The tube 35 will then gradually become conductive following an exponential curve determined by the charging rate of the condenser 44 through the resistors 43 and ment, the output curve of a single stage will remain flat for a length of time determined by the time required for the capacitor 44 to reach the cut-ofl. potential of the tube 35 from the relatively greater negative potential applied to the grid of tube 35 and, after reaching this point, the tube 35 will follow an exponential curve in becoming conductive. The timing of the exponential curve may be varied by changing the capacitor 44 or the resistor 43 or 45 or either of them, while, with any given timing for this time circuit, the length of time duringwhich the tube remains non-conductive after cessation of a signal is determined by the excess of negative voltage applied to the grid of the tube 35. With this arrangement, if the shutter is opened to a given amount and the signal suddenly decreases and thereafter immediately increases to its original value within the time of constant level, the shutter will not change in its position.

When the capacitor 44 has discharged to a potential corresponding to the minimum plate voltage on the tube 30 which is approximately 14 volts the potential on the grid of the tube 30 again takes control and the tube becomes nonconductive. Potential then builds up on the capacitor 44 through the resistor 45 until either the capacitor 44 is charged to maximum potential or a subsequent signal has again triggered the tube. It will be apparent from this that each stage, including the tubes corresponding to the tube 30 and the tube 35, constitutes a separate sawtooth oscillator triggered by the input signal and having a periodicity determined approximately by the capacitor 44 in the resistors 43 and 4B would be a tendency for that stageto turn on,

and off irregularly, and this is prevented by the portion of the circuit next described.

The power supply of 220 volts is applied at the terminals indicated.

The stages B, C, D and E differ slightly from the stage A. In stage B the resistor 50 has a value of 330,000 ohms, the potentiometer has a resistance of 100,000 ohms, and the resistor 52 has a resistance of 56,000 ohms. The resistor 53 has a value of one megohm, resistor 54 6.8 megohms, and the capacitor 55 .025 mfd. as in stage A. ,In the stages C, D and E the resistors 63, I2 and 82 likewise have a value of one megohm and the variable resistors 54, 13 and 03 each have a value of 6.8 megohms maximum. The capacitors'63, 14 and 84 likewise have a value'of .025 mid. In stage C the resistor 50 has a value of 470,000 ohms, the potentiometer 6i 9. resistance of 50,000 ohms, and the resistor 62 a resistance of 10,000 ohms. In the stage D the re- 45 eflectively in parallel. Due to this arrangesister 10 has a value of 470,000 ohms, while the potentiometer 12 has a resistance oi 10,000 ohms and in stage E the resistor 80 has a resistance of 470,000 ohms and the potentiometer 81 a resistance of 5,000 ohms.

It will be apparent from the foregoing that the voltage dividers for applying potential to the grids are chosen according to the signal level to which the several stages are to respond. In

the arrangement; shown, satisfactory triggering levels and ranges for the successive stages on the basis of a maximum level of zero db. (6 m. w. reef.) and a. minimum level of 60 db. may be:

50 db. for stage A, 54 db. to 46 db. db. for stage B, 42 db. to 33 db.

Level 20 db. for stage 0. Range -33 db. to l4 db.

l0 db. for stage D, -16 db. to max. level -4 db. for stage E. 8 db. to max. level The opening time may be 10 to 12 milliseconds and the closing time to milliseconds.

The foregoing levels plotted as input in decibels against output of the D.-C. amplifier in milliamperes is shown in Figure 2.

The tube 96 is preferably of the type known as an RCA VR-105/30, which is a unidirectionally conducting tube establishing a voltage drop of 105 volts, and limits the potential between the midpoint of the resistors 98 and 94 and the lead to the cathodes of the tubes 35, 30, 31, 38, 39 to 105 volts, leaving the balance of potential of 230 volts or volts distributed between the 10 ohm resistor and the 22,000 ohm resistor 92. Due to this arrangement, when one of the gas tubes becomes conducting and plate current starts to flow, since the potential diiierence between the resistor 95 and the cathode lead of the tube 25 is maintained constant at volts, an increase in the drop amounting to approximately volt occurs across the resistor 95 for each stage triggered, thereby decreasing the grid bias applied to the tubes 30, 3|, etc., by a corresponding amount and causing the tubes 30, etc., to remain conductive as long as the input signal does not fall more than one-half volt below the signal which originally turned on that particular stage. Conversely, when the input signal drops more than one-half volt below that which originally turned on the stage, that stage becomes inoperative and remains so until the signal reaches the original trigger level.

The resistor 95 which controls the lock-in voltage would not have a proportionate value in excess of that above assigned to it and it may well on occasion have a lower value. If the lock-in voltage produced by the resistor 05 is too high, it will produce an appreciable difference between the instantaneous and the continuous peak voltage to which the higher stages will respond.

Since each of the stages of the step amplifier is, as pointed out above, substantially a sawtooth oscillator, there is a tendency for the device to produce a buzz which is applied across the resistor 0t and thereby applied to the untriggered stages through the condensers d0, 53, etc., of each of them. The untriggered stages transmit the buzz to the ground noise reduction apparatus. This buzz may be eliminated from the output by the provision of a bypass condenser 0|.

or a shutter such as commercially used in variable density recording, or the output may be applied in an appropriate manner to any other type or ground noise reduction apparatus, volume compressor, volume expander or any equivalent device. Although the output from the step amplifiermight be sufiicient to operate the shutter or other device, the shutter amplifier is necessary in order to isolate the output circuit, as otherwise the inductance, etc, oi the output circuit might form a resonant circuit with the capacitor 91 of the thump filter.

The output from the step amplifier is applied across the potentiometer so which is bypassed by the 1.0 mfd. capacitor 9i. This potentiometer preferably has a resistance of approximately 3,000 ohms and with the capacitor 9i constitutes a thump filter. As pointed out above, the several step amplifier stages open very rapidlypractically at radio frequency. In the application of the output of the step amplifier to ground noise reduction apparatus, it is necessary to'slow this action down to an inaudible frequency, as otherwise the step action of the amplifier will be reproduced as a cliclr or thump. The particular filter arrangement shown permits an opening time of approximately 10 to 12 milliseconds, which is below the normal frequency range transmitted by present theater amplifiers or reproduced by the present commercial types of speakers. Since the output from the step amplifier is directcurrent and at a fairly high positive potential, it is necessary to provide an appropriate potential across th resistors 98 and 99 between the contact to the potentiometer 90 and the grid of the tube its. The potential applied to the control grid of the tube idfi is determined by the potentiometer $8. The tube 900 is preferably RCA type SFS and may be provided with an adjustable cathode bias resistor I of 1500 ohms resistance. The resistor 58 may have a resistance of approximately 10,000 ohms and the tube W3 is preferably the RCA type known as the VR-150/30 which provides a drop of 150 volts. It will be apparent that the tube Mid, the tube tilt, the resistor ifii and the resistor Hi2 form a bridge circuit and the ground noise reduction apparatus is connected across this bridge circuit at HM. In the particular arrangement shown, an increase in speech input across the transformer causes a decrease in potential drop across the resistor fit, thereby decreasing the bias on the tube His and increasing its plate current.

In current commercial practice, the load resistance of a ground noise reduction shutter such as connected at ififi is approximately 70d ohms ancl'it requires a maximum current 01' 30 milliamperes. When the apparatus is properly adjusted, the resistor iiii is so adjusted that there is at least 80 rent through the load when the tube 53 is biased to cut-off. Ther is 150 volts drop across the tube its with a current of 35 milliamperes through it and 5 milliamperer of current passing through the resistor toe to keep the tube 503 in operation. With a supply current of 220 volts there is a 70 volt drop across the resistor Hi2 and a 21 volt drop across the 706 ohm load at M4. The. loltage drop across the tube are is then Hi volts and that across the resistor Elli is 49 volts. When the circuit is thus connected, the control grid of the tube Hill become more positive on increase in output from the speech amplifier and draws current since the voltage across the tube 9% remains constant at 150 volts. s the curmilliamperes cur-.

rent is required to increase with increase in signal current, the shutter may be inserted in place of the resistor l0! and the tube I03 is removed. In this arrangement, the increase in current through the tube )0 causes a correspondingincrease in current through the load.

, It will be apparent to those skilled in the art that the invention is not limited to the exact construction before described but that the construction and characteristics or the apparatus may be greatly varied within the scope of the invention. In the apparatus above described, it a gradually increasing signal is applied to the speech amplifier, the step amplifier will increase its output step by step as the signal gradually increases until the maximum output is reached. If the signal is then completely removed all the timing circuits which are similar will cause the output of the several stages of theistep amplifier to decrease simultaneously, and th output will drop to zero at the time determined by any one of the timing circuits. It the signal decreases gradually, the output will decrease step by step at the end of the closing time determined by the timing circuit of each step.

The characteristics of the opening timing circuit 90, 9| may be varied as desired according to the characteristics of the apparatus with which the amplifier is to be used, but it should be kept in mind in making any such variation that the ground noise reduction apparatus should open as rapidly as possible without producing any audible thump.

I claim as my invention:

1. In a trigger t'ype amplifier, a gas type trigger tube including a control grid, a vacuum type amplifier tube, means for applying a plate potential to said trigger tube and a grid potential to said amplifier tube, a timing circuit including a capacitor shunted by a resistor connected between the plate and cathode of said trigger tube, means for applying a grid bias to said trigger tube, and means for applying a signal to the grid of said trigger tube.

2. In a trigger type amplifier, a gas type trigger tube including a'con'trol grid, a' vacuum typo amplifier tube, means for applying a plate potential to said trigger tube and a gridpotential to said amplifiertube, a'timing circuit including a capacitor shunted by a resistor connecind between the plate and cathode of said trigger tube, means for applying a grid bias to said trigger tube, means for applying a signal to the grid of said trigger tube, and means for degregsing said grid bias on the triggering of said 3. In a trigger type step amplifier, a plurality of step 'stagesin parallel, each including a gas type trigger tube 'ncluding a controLgrid, a vacuum type amplifier tube, means for applying a, plate potential to said trigger tube and a grid potential to said amplifier tube, and a timing circuit including a capacitor shunted by a resistor connected between the plate and cathode of said trigger tube.

4. In a trigger type step amplifier, a plurality oi step stages in parallel, each including a gas typ trigger tube including a control grid, a. vacuum type amplifier tube, means for apply ing a plate potential to said trigger tube and a grid potential to said amplifier tube, a timing circuit including a capacitor shunted by a resister connected between the plate and cathode of said trigger tube, means for applying a grid bias to said trigger tube, and means for applylug a, signal of dlflerent amplitude to the grid v of each or said trigger tubes.

5. In a trigger type step amplifier, a plurality oi step stages in parallel, each including a gas trigger tube including a control grid, e. vacuum type amplifier tube, means fox-applying type trigger tube including a, control grid, u vacuum type amplifier tube, mean for a l i a. plate potential to said trigger tube and a grid potential to said amplifier tube, e, timing circuit including a capacitor shunted by a resistor connected between the plate and cathode of said. trigger tube, means for applying a grid bias to said trigger tube, means for applying a signal of s plate potential to said trigger tube and a. grid potential to said amplifier tube, a timing circuit including a capacitor shunted by a. resistor connected between the plate and cathode of said trigger tube, means for applying a grid bias to said trigger tube, means for applying a signal oi dlflerent amplitude to the grid of each of said trigger tubes, and means for decreasing said grid bias on the triggering of each of said tubes.

6. in a, trigger type step amplifier, a plurality of 'etep stages in porellel, each in u gee difierent amplitude to the grid of each oi seid trigger tubes, and means for decreasing said grid bias on the triggering of each of said tubes, said bla's-clecreeslng me including a. voltage divider having in sequence vacuum tube plate voltage, trigger tube plate voltage and trigger tube grid voltage sections, the cathodes oi the vacuum tubes being connected to the positive end of the trigger tube plate voltage section, and e constant voltage device connected across the trigger tube plate voltage and trigger tube grid ac voltage portions of said divider only. 

